In the wake of the collapse of Enron and the charges against the accounting firm of Arthur Anderson, Congress passed the Sarbanes-Oxley Act of 2002, which included a requirement for the GAO to study public accounting firms:  Mandated Study on Consolidation and Competition

While done years ago, that study is still relevant today.  See,for example, the Wall Street Journal’s article on August 17, 2011:  “U.S. Auditing Board to Study Accounting-Firm ‘Term Limits‘”